The present invention relates to a ballast for at least one electric incandescent lamp, having at least two switching elements that are coupled to one another with the formation of a first tie point, a drive circuit with at least one output for driving the at least two switching elements with the aid of a drive signal that is characterized by a pulse duty factor and an operating frequency, and with an input for a dimming signal, and a power transformer whose primary side is coupled to the first tie point, it being possible to connect the at least one electric incandescent lamp to the secondary side. It also relates to a method for operating a ballast for at least one electric incandescent lamp, in which the ballast comprises at least two switching elements that are coupled to one another with the formation of a first tie point, a drive circuit with at least one output for driving the at least two switching elements with the aid of a drive signal that is characterized by a pulse duty factor and an operating frequency, and with an input for a dimming signal, and a power transformer whose primary side is coupled to the first tie point, it being possible to connect the at least one electric incandescent lamp to the secondary side, the first step in the method being to apply a dimming signal to the input of the drive circuit, after which the drive circuit generates a drive signal as a function of the dimming signal.
The present invention relates to the dimming of electric incandescent lamps that are driven via what is termed an electronic transformer. In order to control such an electronic transformer via a digital or analog interface, it is necessary to be able to set the voltage at its output terminals over a wide voltage range. It is necessary in this case to be able, in particular, to set the voltage very accurately such that the light flux characteristic prescribed for dimming in what is termed the DALI Standard is observed, and thus all lamps of a group of electronic transformers appear equally bright even in the dimmed state. That is to say, different electronic transformers that drive different incandescent lamps must provide the same output voltage as precisely as possible in response to a dimming signal. This is necessary, since brightness differences are visible with particular effectiveness precisely in the dimmed state. A pulse-width method (asymmetric duty cycle) in which the power switches switch alternately in the half bridge is particularly advantageous for the output voltage setting required for dimming. In this case, the switch-on time of one switch is lengthened at the expense of the switch-on time of the other switch. The operating frequency remains constant in this method, and only the switch-on conditions of the switches change.
Although FIG. 1 shows an exemplary embodiment of the ballast according to the invention, it can also be used to describe the prior art: two power switches are denoted in FIG. 1 by T1 and T2, and are driven by a drive circuit 10 that is coupled, for its part, to the output of a dimmer 12, the dimmer 12 applying a dimming signal Ud to the drive circuit 10. The supply voltage of the two switching elements T1 and T2 is what is termed the intermediate circuit voltage Uz, which is provided here across a capacitor Cz. The tie point VP1 of the two switching elements T1 and T2 is connected to the primary side L1 of a transformer xc3x9c. The other terminal of the primary side L1 of the transformer xc3x9c is connected to the tie point of two coupling capacitors Ck1, Ck2 that are connected with the other electrode to the capacitor Cz. At least one incandescent lamp LA1, LA2 is connected to the secondary side L2 of the transformer xc3x9c.
FIG. 2 shows the profile of the voltage U1 on the primary side L1 of the transformer xc3x9c, the pulse duty factor between the switch-on time t2 of the top switch T2 and the switch-on time t1 of the lower switch T1 being varied such that t1 less than t2.
The root-mean-square value of the secondary-side output voltage U2eff is calculated as:       U          2      ⁢      eff        =                    U        z            N        ·                            d          ·                      (                          1              -              d                        )                              .      
Here, d stands for the pulse duty factor, that is to say d=t1/T, wherein T=t1+t2 denotes the period of the primary-side voltage U1 across the power transformer xc3x9c, the reciprocal of which corresponds to the operating frequency fs. As already mentioned, Uz corresponds to the intermediate circuit voltage, and N to the number of turns per unit length of the power transformer xc3x9c. The range 0 to 0.5 is permissible for d. It is usual in the case of dimming to dim down to a minimum value of approximately 1% of the maximum light flux. Very small pulse duty factors in the range of d=0.03 to 0.05 are achieved in this case. The switch-on time t1 is very small in this case. Consequently, the spectrum of the output voltage U2 is shifted drastically in the direction of higher frequencies. The voltage at the output terminals is affected thereby only to a small extent, the more so as the additional voltage drop across the transformer leakage inductance can be compensated by appropriate switching measures known from the prior art.
However, a particular disadvantage arises in the case of the use of a ballast to operate a plurality of incandescent lamps. Specifically, the shifting of the spectrum gives rise to a substantial inductive voltage drop across the supply leads to the incandescent lamps. This voltage drop can be up to several volts in the case of small pulse duty factors d and at customary operating frequencies from 30 kHz to 50 kHz. If a plurality of incandescent lamps in parallel connection are then operated by a ballast, they therefore burn, depending on the design of the lamp supply lead, that is to say, in particular, in accordance with the inductance of the lamp supply lead, which is determined essentially by the length of the lamp supply lead, with a clearly visible difference in brightness. With reference to the mode of operation mentioned first, in which individual incandescent lamps are driven by dedicated transformers, this likewise holds when these are dimmed jointly via a common digital bus.
It is therefore the object of the present invention to develop a generic ballast and the generic method in such a way that differences in brightness during operation of a plurality of incandescent lamps by such a ballast and during operation of various incandescent lamps having various electronic transformers, which are jointly dimmed via a common bus are avoided to a very large extent.
The above object is achieved according to a first aspect by virtue of the fact that the drive circuit is designed to vary the pulse duty factor and the operating frequency of the drive signal as a function of the dimming signal.
In accordance with a further aspect, the object is also achieved by a generic method in which in the second step the drive signal is generated by varying the operating frequency and the pulse duty factor as a function of the dimming signal.
This measure permits a substantial reduction in the operating frequency, as a result of which it is possible to considerably reduce or even entirely avoid a shifting of the spectrum of the output voltage during dimming. Furthermore, losses in the case of generic circuits are proportional to the operating frequency. The lowering of the operating frequency therefore results in fewer losses in the case of the invention. A further advantage with regard to the electromagnetic compatibility arises from the fact that fewer high-frequency components occur on the supply leads to the electric incandescent lamps. In addition, it is possible to select a more cost-effective timer for implementing the pulses, since the generation of narrow pulses is more difficult to implement technically, and the pulses can be selected to be wider in the case of the present invention. Consequently, the ballasts according to the invention can be implemented very cost-effectively owing to the reduced requirements of the timers.
In a preferred embodiment, the drive circuit is designed in the event of variation in the strobe signal to continue to take account of the supply voltage for the two switching elements and/or the current through the at least one incandescent lamp and/or the voltage on the secondary side of the power transformer. This measure permits the signal driving the switching elements to be even better optimized with regard to the efficiency of the power transformer.
The drive circuit is preferably designed to vary the operating frequency as a function of the pulse duty factor. Since the pulse duty factor and operating frequency are tuned to one another, and one or more relevant variables are taken into account in the definition of the pulse duty factor, this is a particularly simple possibility of fixing the operating frequency suitable therefor. In particular, the operating frequency thereby no longer needs to be calculated separately, but can be read out in a fashion suitable for the pulse duty factor, for example by using a look-up table.
According to the invention, the drive circuit is designed, in the event of a dimming signal that is correlated with a relatively low power to be transmitted by the power transformer, to reduce the operating frequency by comparison with the operating frequency in the event of a dimming signal that is correlated with a relatively high power to be transmitted by the power transformer.
The drive circuit is preferably designed, in the event of a dimming signal that is correlated with the maximum power to be transmitted by the power transformer, to generate a drive signal in the case of which the operating frequency is a maximum and, in the event of a dimming signal that is correlated with the minimum power to be transmitted by the power transformer, to generate a drive signal in the case of which the operating frequency is a minimum.
The minimum operating frequency of the drive signal is preferably determined in such a way that saturation still does not occur in the power transformer. The drive circuit can be designed to simplify the control, the drive circuit is designed to vary the operating frequency of the drive signal as a function of the dimming signal in a continuous fashion, a stepwise fashion and/or in accordance with the partly linearized curve profile.
The drive circuit can also be designed to reduce the operating frequency of the drive signal in such a way that the voltage-time area at the power transformer remains substantially constant. If the voltage-time area is selected in this case such that the transformer is operated with optimum efficiency in the case of maximum output power, this permits one or more incandescent lamps to be operated with very low losses in the ballast.
However, the voltage-time area can also be selected in such a way that the power transformer is operated at the saturation limit. The frequencies in the output spectrum thereby turn out to be minimum, and so the results turn out to be minimum with regard to electromagnetic compatibility, a timer that is to be selected, and power losses.
The ballast can comprise two switching elements in a half-bridge arrangement or four switching elements in a full-bridge arrangement.
Further advantageous embodiments follow from the subclaims.